Now English isn't my first language but when you say joint i get that as sharing. Will she be shared or will there be option for separate actions?
Well Myriam could definitely be spit-roasted by Cassius and Hammer, and we had a discussion a few months ago, about whether she could be double-penetrated by them. And it is possible apparently, because dogs can lie on their backs. So Cassius could theoretically lie on his back, and have Miriam ride him in her pussy, whilst Hammer fucked her ass from behind.
A dog wouldn't typically do that, so it seems very unlikely. And it probably wouldn't be a comfortable position for Cassius, even if Myriam could make him do it. But hey this is a fantasy, where Myriam can be ravaged mercilessly by Hammer's monstrous cock, without suffering permanent interior damage. So I'm sure Cassius would manage fine?
Personally, if you're gonna have a scene with both of them together, why wouldn't you have them both fuck Myriam together? This might be a one-off situation, where you get both of them, in the same place at the same time. So why wouldn't Myriam take advantage of that?
If you do it singly, then Cassius would have to go first. Because after Hammer, his new favourite hole would be gaped so wide, he probably wouldn't be able to feel anything.